Combustion gases from, for example, coal-fired power plants, can be a significant source of the release of heavy metals into the environment. Similarly, gases from crematoria can contain mercury emissions from tooth fillings, and possibly other heavy metals. Other sources of metal emissions include cement kilns, chlor-alkali cells and artesianal gold-mining. There is a rapidly growing realisation that effective technology is required to permit the removal of such heavy metals from oxygen-rich gas streams such as fossil fuel combustion gases which are released directly into the atmosphere.
In addition to removal of these heavy metals from directly released, oxidising, gas streams, there is a similarly increasing requirement for their removal from process gas streams, where their presence can, for example, be deleterious to subsequent downstream processes, especially those involving catalysed change of the gas stream composition or use of the gas stream for generating power. For example, currently in the United States, the potential use of huge supplies of coal to generate hydrogen-rich fuel gas (or syn-gas) via coal gasification processes is receiving growing attention as the price of imported oil increases. Coal contains toxic heavy metals in varying proportions and the ability to remove these metals cheaply and efficiently from the resultant fuel gas would be of great commercial benefit.
The conditions encountered in such combustion and fuel gases are, however, technically challenging. In the case of coal combustion gas streams, for example, these include large amounts of flyash, the presence of toxic metals such as mercury and arsenic and toxic gases such as sulphur and nitrogen oxides, potentially corrosive halide species, all present in huge volumes of gas at atmospheric pressure. In the case of the coal gasification-derived fuel gas, for example, the gas leaves the gasifier at high temperatures, for example >700° C., and high pressures, for example up to 70-80 bar. Current technology, for example as used in a plant in the United States, requires that the gas be cooled to near ambient temperatures for removal of toxic metals such as mercury via adsorption onto carbon beds. Downstream processing can then require that the cleaned gas be re-heated. It has previously been estimated that the ability to remove toxic metals from the fuel gas stream at elevated temperatures could result in around a 3% improvement in energy efficiency for the plant, because it is thermodynamically undesirable to cool the gases to allow some methods of heavy metal removal.
The US Department of Energy's National Energy Technology Laboratory has identified, in U.S. Pat. No. 7,033,419, that mercury can be removed from fuel gases using precious metal-containing absorbents. There remain, however, technical challenges to be overcome before a commercial-scale mercury absorbent can be developed.
U.S. Pat. No. 4,814,152 (Mobil Oil Corp) discloses a process for removing mercury from a gas using an inert support which contains at least about 5 wt % of elemental sulphur, and a catalyst (which may be platinum or palladium) which catalyses the reversible reaction 2Hg+S2=2HgS at a temperature of not more than 170° C.
U.S. Pat. No. 5,601,701 (Institut Francais du Petrole) discloses the removal of mercury from hydrocarbon fractions, using a catalyst bed and a mercury retention bed. The catalyst may be a partially-sulphided supported metal, preferably nickel or an association of nickel and palladium. If palladium is present, it should not exceed 0.2% of the catalyst. The catalytic process operates at temperatures of 120-250° C. The mercury retention bed which follows the catalyst bed is suitably composed of copper sulphide, tin dichloride or potassium iodate.